Molecular electronic devices offer the possibility of vastly increased circuit element density, and thus computing power, in future integrated circuits. Moreover, nanoscale molecular electronics is a potential platform for quantum computing, and for biosensing and genetic screening, among others possibilities. A large problem at present in the field of molecular electronics concerns making electrical contact to nanoscale objects such as molecules, which typically range in size from about 1 to about 10 nanometers. The future of molecular electronics will depend on the ability to contact molecules to the outside world in a reliable fashion. However, until now, there has been no known reliable and cost-effective method of creating such nanoscale electrode gaps, especially in the range of gap sizes between about 1 nm and 20 nm.
The typical route to making such an electrical contact is to fabricate macroscopic electrode structures that successively connect to smaller and smaller electrodes, ending in a nanoscale electrode gap. After electrode fabrication, this gap is bridged by (connected to) a molecule of interest, forming a continuous electrical circuit. That is, a molecule or assembly of molecules is arranged to be situated in the gap so formed in such a way as to form a continuous electrical connection across the gap. This is achieved by known means, such as electrostatic trapping, or chemical binding.
A technology currently used to fabricate gaps is electron beam lithography (EBL, or e-beam). In this technology, a focused beam of electrons is used to expose an appropriate polymeric “resist” material coating a substrate, either weakening (scission) or strengthening (cross-linking) chemical bonds in the exposed region of the polymer, thus rendering the polymer either soluble or insoluble, respectively. This allows one to wash away the weaker resist material (either the exposed or the unexposed region), exposing the area underneath for future deposition of material. Creating feature sizes below 1000 nm generally requires EBL, while those larger than 1000 nm can be fabricated with less expensive, photolithographic techniques.
With a large amount of effort, the best EBL systems theoretically may be capable of defining nanostructures, including gaps between nanowires, although not with a high degree of reproducibility. Even an expert user, however, will have great difficulty achieving reliable electrode gaps of a small size on even the best available commercial instruments. Typical gap sizes may be between 20 and 200 nm. However, the typical cost of an EBL system with such capabilities is currently between one million and fifteen million dollars, making this a very expensive technique. Feature sizes, including electrode gaps, smaller than 20 nm are produced with very low yields, of the order of 10%.
Another technology that exists for the fabrication of nanoscale electrode gaps is “electromigration”. In electromigration, an electrode is formed having a continuous metallic nanowire defined on a substrate with the added feature of having a narrow constriction along the length (usually in the middle) of the nanowire. This is achieved by using e-beam lithography to fabricate the thin metal electrode on a substrate. After fabrication is complete, electric currents are passed through the electrode in increasing amounts to induce heating. By virtue of possessing a smaller cross-section, the constricted area has a larger electrical resistance than the remaining unconstricted nanowire, and accordingly more heat is dissipated at the constriction. This occurs because the rate of heat dissipation is directly proportional to the resistance. Upon applying a high enough current to attain a threshold value, the heat generated thermally induces the nanowire to break at the narrow constriction, creating a small gap. This is similar to the mechanism of blowing a household fuse with an overload of electrical current.
Electromigration has been shown to reliably produce electrode gaps of up to 1 nm in size in nanoscale wires. However, once the gap opens, no more current can flow and no more shaping of the gap is possible. Thus, larger gaps of up to 10 nm may not be possible by this method because not enough heat is generated to facilitate an increase of gap size after the thermally induced break occurs. Accordingly, the gaps produced by this technology are limited to about less than 1 nm in size.
In addition, this may not be a highly reproducible method. Although gaps of less than 1-nm in size have been reportedly reproduced, other groups have reported wide variability, with yields of on the order of 10%. Fabrication of metallic electrodes with nanometer separation by electromigration, Hongkun Park, Andrew K. L. Lim, A. Paul Alivisatos, Jiwoong Park, and Paul L. McEuen, Appl. Phys. Lett. 75, 301 (1999). The Kondo effect in C60 single-molecule transistors, L. H. Yu and D. Natelson, Nano Lett. 4, 79 (2004). The latter reports the aforementioned variability. Thus, this technology has limitations due to the gap formation being self-limiting to the few nanometer range and to its apparently low reproducibility.
U.S. Pat. No. 6,737,286 discloses an apparatus and a method for fabricating self-terminating molecular-scale and atomic scale contacts and gaps between electrodes, wherein a pair of electrodes separated by a gap are subjected to an electrical etching process that decreases the gap between them.
U.S. Pat. No. 6,383,923 discloses a circuit device comprising two or more circuit sections vertically interconnected with nanowires for nanoscale circuit interconnections and tactile sensor devices. The nanowire contacts are grown in dissolvable or removable substrate.
However, none of the above address the primary concern in electrode preparation, which is that the separation gap between two electrodes be of the proper size to successfully bridge biomolecules such as DNA strands of appropriate length for biosensing. The lengths of these DNA strands are about 10 nm, or about 30 base pairs. Strands much shorter (˜1 nm) are non-specific and thermodynamically unstable, whereas strands much longer (˜100 nm) are statistically less likely to hybridize in the sensor configuration. Although electrode gaps down to 10 nm have been reported, there remains quite a lot of variation in reproducibility and in the gap size, particularly with gaps in the 20 to 40-nm range. Direct measurements of electrical transport through DNA molecules, D. Porath, A. Bezryadin, S. de Vries, C. Dekker, Nature 403, 635 (2000). Even with a multimillion dollar e-beam writer, the yield of an ideal 10 nm electrode separation may be low. E-beam fabrication of electrodes for transport measurements across single molecules, J. Moser, R. Panepucci and M. J. Naughton, National Nanofabrication Users Network, Cornell Nanofabrication Facility 2000-2001 Research Accomplishments, p. 132-133 (2001).
Therefore, there is a need for a significant innovation in producing nanoscale electrode gaps of accurately controlled or predetermined size. Further, there is need in the art for a method to fabricate 10 nm structures in a reliable fashion. Thus, an apparent gap exists in the formation of nanogaps that can be formed: about 1 nm by electromigration and about 100 nm by e-beam in a cost-effective and highly reproducible manner.